1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device and a driving method thereof that can prevent a color distortion phenomenon.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device controls the light transmittance of liquid crystal cells in accordance with video signals to display pictures. A LCD device of an active matrix type having a switching device provided for each liquid crystal cell is suitable for displaying moving pictures. A thin film transistor (TFT) is generally employed as a switching device for the active-matrix type LCD.
FIG. 1 is a schematic sectional view illustrating a structure of a liquid crystal display (LCD) panel of a related art LCD device.
Referring to FIG. 1, in the LCD panel, color filters 6 and a common electrode 4 are disposed on a upper substrate while a pixel electrode 10 is provided on a lower substrate 12. A liquid crystal 8 is provided between the upper substrate 2 and the lower substrate 12. A white lamp 14 acting as a backlight source is provided under the lower substrate 12. On the upper substrate 2, black matrices (not shown) are provided among the red, green and blue color filters 6. Each of the upper substrate 2 and the lower substrate 12 has an alignment film (not shown) formed on the interface surface that is in contact with the liquid crystal 8. On the lower substrate 12, gate lines and data lines perpendicularly cross each other, and a TFT is provided at each crossing of the gate lines and the data lines. The TFT provides a channel between a source terminal thereof connected to the data line and a drain terminal thereof connected to the pixel electrode 10 in response to a scanning pulse applied, via the gate line, to a gate electrode thereof, thereby applying a data signal to the pixel electrode 10. The pixel electrode 10 is provided within a pixel area that is enclosed by the gate lines and the data lines.
In such a LCD panel, a voltage is applied to the white lamp 14 to continuously irradiate a white light onto the LCD panel. When the white lamp is turned on, the scanning pulse turns on the TFTs to simultaneously apply red, green and blue data signals to the corresponding liquid crystal cells. As a result, the white light (backlight) irradiated onto the LCD panel passes through the liquid crystal cells and the red, green and blue color filters 6, and thus becomes a red light, a green light and a blue light. The LCD panel displays a desired color by a combination of the red, green and blue lights.
The LCD panel according to the related art has a drawback in that the backlight looses approximately one-third of its intensity while passing through the color filer 6, which raises a problem of low brightness. In order to solve such a problem, a field-sequential driving method has been suggested.
FIG. 2 is a schematic sectional view of a LCD panel driven by a field-sequential driving method according to a related art.
Referring to FIG. 2, the LCD panel has no color filter, but includes a red lamp 36R, a green lamp 36G and a blue lamp 36B. Black matrices (not shown) are provided on a upper substrate 22 of the LCD panel. Each of the upper substrate 22 and a lower substrate 32 has an alignment film (not shown) formed on the interface surface that is in contact with a liquid crystal 28. On the lower substrate 32, gate lines and data lines perpendicularly cross each other, and a TFT are provided at each crossing of the gate lines and the data lines. The TFT provides a channel between a source terminal thereof connected to the data line and a drain terminal thereof connected to a pixel electrode 30 in response to a scanning pulse applied, via the gate line, to a gate electrode thereof, thereby applying a data signal to the pixel electrode 30. The pixel electrode 30 is provided within a pixel area that is enclosed by the gate lines and the data lines. In such a field-sequential driving method, a lamp driving voltage is sequentially supplied to the red lamp 36R, the green lamp 36G and the blue lamp 36B to sequentially irradiate red, green and blue lights to the LCD panel.
According to such a field-sequential driving method in which red, green and blue lights are sequentially irradiated onto liquid crystal cells during one frame interval, light transmittance becomes higher in the order of the red light, the green light and the blue light due to a cumulative response characteristic of the liquid crystal, thereby raising a color distortion problem. In other words, the cumulative response characteristic of the liquid crystal distorts the light transmittance, and when the red, green and blue lights from the red, green and blue lamps 36R, 36G and 36B pass through the liquid crystal, the distorted light transmittance distorts the color of the liquid crystal cell.
Such a color distortion phenomenon distinctly appears when a yellow color is displayed. According to such a field-sequential driving method, a yellow color can be displayed by sequentially driving a red light and a green light. Because of the time resolving power of human beings, the sequentially driven red and green lights can be recognized as a yellow color. Because the green light is transmitted to the liquid crystal cell after the red light when displaying a yellow color, the transmittance of the red light becomes lower than that of the green light due to the cumulative response characteristic of the liquid crystal. In this case, because the green light contributes to the brightness of the liquid crystal cells twice as much as the red light (wherein brightness contribution degree has generally a relationship of R:G:B=3:6:1), a yellow color close to a green color is displayed on the LCD panel, thereby raising a color distortion problem.